It is useful to be able to determine the length of time for which a package containing perishable goods has been opened. To this end, many different sensors for detecting oxidising agents, and in particular for detecting oxygen, have been produced. Several of these sensors have been adapted for attachment to packages containing perishable goods. For example, GB 2419868, FR 2836677, WO 2006/077413, GB2344101 and US 2006/0110835 disclose the use of oxygen sensitive dyes that are covered by a seal, the seal being broken by the opening of a package, and the dye changing colour over a set period of time to indicate the duration for which the package has been opened.
WO 03/021252 (incorporated herein by reference) discloses a sensor for oxidising agents which is activated using UV light. This patent application discusses the use of a particular chemical composition which can be in the form of an ink, and which may be printed onto a variety of supports. However, the indicator disclosed is necessarily sensitive to oxygen. It is not apparent from reading the application how such an indicator may be effectively integrated with, or printed onto, packaging in the reduced form. In addition, it is unclear as to how such an indicator may be applied in the reduced form in atmospheric conditions.
The composition described in WO 03/021252 comprises at least one redox-sensitive material, at least one semiconductor material and at least one electron donor. The intimate contact of the various components of the indicator allows the redox sensitive material to undergo a redox reaction wherein there is a transfer of electrons from the photogenerated reduced form of the semiconductor material to the redox sensitive material.
The redox-sensitive material can be a dye such as a thiazine dyestuff, an oxazine dyestuff, an azine dyestuff, a triphenylmethane dyestuff, an indophenol dyestuff, an indigo dyestuff, viologen and/or mixtures thereof.
The semiconductor material has the ability to form an excited electronic state that is sufficiently oxidising to oxidize the sacrificial electron donor and has a reduced form that is able to reduce the redox sensitive material.
The semiconductor material may be an oxide of titanium (such as titanium (IV) oxide; TiO2, and strontium titanate; SrTiO3), tin (such as tin (IV) oxide; SnO2), tungsten (such as tungsten (VI) oxide; WO3) and zinc (such as zinc (II) oxide; ZnO) and mixtures thereof.
The electron donor has the ability to donate electrons, preferably irreversibly. Typically, the electron donor is a mild reducing agent. The electron donor may, for example, be an amine (e.g. NaEDTA and TEOA), reducing saccharide (such as glucose and fructose), readily oxidisable polymer (such as polyvinyl alcohol), and other general anti-oxidant (such as ascorbic and citric acid) or easily oxidizable material (such as glycerin) and/or mixtures thereof.
The indicator may further comprise a binder which binds all the components together. The binder may be a polymeric material such as gelatin, hydroxyethyl cellulose (HEC), polyvinyl alcohol (PVA), ethyl cellulose (EC), cellulose acetate (CEA), polypyrolidone (PVP), polyethylene oxide, and polymethylmethacrylate (PMMA).
Many modern day printing processes, and many types of modern packaging, require the use of organic solvent based inks. Whilst it is stated in WO 03/021252 that the indicator compositions can be combined with an organic solvent to provide an ink or printable solution, the inventors have found that the redox active species tends to leach out of such solutions. For example, the polymers described as binders are generally water soluble (such as polyvinyl alcohol), as are the redox active species (such as methylene blue). When these compositions are added to organic solvents the dye (redox active species) and the polymer do not dissolve well. Furthermore, on exposure of the indicator compositions described in WO 03/021252 to water or moisture (as is normally present to at least some degree in organic solvents and in the atmosphere), the dye in particular tends to leach out of solution. As the dye leaches out of solution the ink degrades and becomes much less effective (less coloured) and more difficult to bleach. Furthermore, the lifetime of the ink becomes more difficult to predict, limiting the use of the ink in, for example, time temperature indicators. Therefore, the indicator compositions described in WO 03/021252 tend to break down on addition to an organic solvent, and on contact with water. Such compositions are therefore inherently unstable.
Generally a composition comprising the indicator compositions of WO 03/021252 will be unstable upon addition to, for example, food packaging. Typically, such a composition will break down upon addition to food packaging as the dye contained therein, upon contact with water contained in the food packaging or the atmosphere, will tend to leach out of the composition. Furthermore, the redox active species can leach out of solution following storage under ambient conditions, meaning that a solution of the composition is not suitable for use in printing.
The inks and indicator compositions described in WO 03/021252 are therefore not suitable for use in many modern day printing processes, which require the use of organic solvent based inks, or in the application to many types of modern packaging, which require that the compositions are stable (i.e., resistant to leaching) on exposure to water. Moreover, it is not apparent how a charged, water soluble, redox active species (such as methylene blue) may be incorporated with a high degree of permanence into an organic solvent soluble indicator composition suitable for use in many modern day printing processes.
Furthermore, it can be impractical to activate indicator compositions (incorporated into inks) after they have been printed and sealed on a substrate, such as packaging. For example, it can be difficult to expose the printed indicator composition to the correct amount of UV light in a controlled fashion, particularly as most printing processes do not have the time window to allow such exposure. Also, exposing the indicator composition to UV light after printing and sealing requires additional specialised equipment to be used in the printing process, and makes the process more complicated and time-consuming. Known UV activatable inks are unsuitable for activation before printing as they will change colour (i.e., oxidise) before or during the printing process.
Therefore, it is an object of the present invention to obviate or mitigate at least some of the disadvantages of the prior art.
A further object of the invention is to provide a method for applying an oxygen sensitive ink to a printing substrate.
A still further object of the invention is to provide an indicator ink that can be used in modern day printing processes.